US2158065A - Art of producing magnetic materials - Google Patents

Description

Patented May 18, 1939 "PATENT OFFICE in or rnonocme MAGNETIC Muslims Guerney H. Cole and Robert L. Davidson, Middlctown, Ohio, assignors to The American 3011-. ing Mill Company, Middletown, Ohio, a corporation of Ohio No Drawing. Application January 9, 1935, Serial No. 1,060

8 Claims.

Our invention relates to the manufacture of silicon steel for electrical uses. Its specific object is the provision of silicon steel sheets for power and distribution transformer use, which have the highest possible permeabilities in a given direction coupled with unusually low core loss also in the same direction. In magnetic materials of this class, the endeavor has for a long time been to improve the core loss ;'but the exigencies of transformer design are such that no further substantial economies can be made therein through lessened core loss, unless substantial improvement can be made in the permeability of the material within the transformer range of inductions.

It has been proposed in improving the permeability of silicon steel to cold work the material and then heat treat it with improvement in the grain size of the product. It has also been observed that even more drastic treatment in cold rolls followed by heat treatment will produce a greater improvement in permeability. One worker has stated that the preferred orientation of crystal lattices resulting from the more drastic cold rolling is the factor which appears to aca count for the improved permeability in the product. Another worker, who has relied upon the drastic cold working, has stated his belief that random orientation results therefrom. Workers in this field have not understood the true situation as we have developed 'it according to our invention, and have set forth as important in connection with the operations certain matters as to specific heat treatments, specific amounts of drastic cold rolling of silicon steel between annealings and the like, which do not, according to our invention, constitute all of the factors of indices; but the teachings of the art have not been sufllcient to enable one in a regular commercial manner to produce silicon steel sheets which are orientated so that the so-called directions of substantially all crystals are parallel to each other and parallel to a face of tl'ie sheet as indicated, for example, by the diffraction of X-rays. It is convenient to refer to this type of orientation as a twin derivative orientation. -'I'he prior; art appears to have made no teachings which enable one consistently to go beyond that degree even of preferred orientation which may be called ordinary cold rolling orientation, nor has it enabled one to regularly secure the type of orientation mentioned above in any substantial degree at all.

It will be understood that ordinary cold rolling orientation, in so far as it is.a preferred orientation, is such that a direction of the crystals is parallel to the direction of rolling and a [100] face of the crystals is parallel to a face of the sheet.

Our process is operative not only for-siiicon steels but also for any other alloyed irons or steels having the same crystal characteristics.

We attain the objects of our invention which have been referred to hereinabove, or which will be apparent to one skilled in the art upon reading these specifications, by that series of process steps of which we shall now describe as an exemplary embodiment, it being understood that our invention is not limited thereto.

In the practice of our invention, we prefer to start with a magnetic material having the following chemical analysis:-

Silicon s 2.90 to 3.30 Carbon 0.02 or under.

We prefer also to have:

Manganese 0.00 to 0.09 Sulphur 0.03 or under.

The silicon ranges given are not limiting; and ,the silicon may be carried substantially above and below these values without departing from our invention. The range given, however, is a convenient working range, with tolerances wideenough to allow for ordinary variations in the alloy, and with a silicon content which permits drastic cold workings without difliculty.

The carbon content is important from the standpoint of core loss, and we desire to finish with a material containing 0.008% carbon or less in order to get the extremely low core loss which we obtain. We have set forth an initial carbon value of 0.02% because it is convenient to make materials of this analysis, and, the an nealing treatments described, carried on in accordance with the usual technique, and practiced upon the materials when covered with a suitable scale or oxide, will be effective to reduce the carbon content to 0.008% or less. In attempts to eliminate carbon during heat treatments. gauge and width and the spacing of individual pieces have an important effect; and where more drastic treatments are desired, it will be effective to separate the sheets or layers by suitable means during the annealing, and/or to substitute box anneals for open anneals, and to control the atmospheresduring annealing. An effective practice is to anneal in a changing atmosphere of so nitrogen. Of course, it is possible to start with a. steel or iron containing initially not more than substantially 0.008% of carbon, such as steel or iron made in the electric furnace, and process it without nltintioll to further reduction of carbon content.

In carrying out our process we take hot rolled stock, give it an initial heat treatment, and carry it down to gauge in two or more drastic cold workings, with an intervening annealing or annealings; and we give it a final heat treatment, as will hereinafter be described.

In our exemplary embodiment, ingots 18 by 39 inches are hot rolled to slabs, say, 3 by 20 by 72 inches, and are heated for 2V hours at approx mately 1800 to 2200 F. This material is then hot rolled to inch in thickness in five passes on a universal mill, and is further hot rolled to, say, 0.105 inch in thickness, through four tandem mills. preferably finishing the material at a temperature about 1500 F.; although it may be finished at different temperatures if desired. In this way the raw material for our proc ess may be formed; but it will be clear that it may likewise be formed in other ways.

This material, 0.105 inch thick, is slit and box annealed in coils at 1400* F., for 24 to 36 hours, after which it may, if desired, be cooled quickly or even quenched, [or improvement in ductility. Or it may be cooled slowly, say, at the rate of F. per hour to 1100" F., at which point the boxes are uncovered and the coils allowed to cool in less than one half hour to 500" F. The material is then pickled.

This initial heat treatment appears to be desirable chiefly as a safety factor because in many instances we have attained our results without using it, and it is employed mainly because the product is uniformly successful when it is produced as described. Merely finishing the material on the hot mills at very high temperatures, while giving satisfactory results in some instances does not give as uniform results as a controlled heat treatment. Again, while in many instances open anneals may be employed, yet these do not appear to give consistent results on all batches. We have found, however, that a box annealing does serve to fit the material for the subsequent steps of our process in a way that gives commercially consistent results.

Our next step is to give the material a drastic cold reduction. In the particular embodiment described we reduce the material from 0.105 inch to 0.029 inch, which is a reduction of the order of 70%. This percentage of reduction may be departed from considerably, but drastic cold rolling of more than 40% appears to be commercially, if not absolutely, necessary; and the cold rolling reduction should not exceed 85%. We prefer to carry down the material in a series of rolling stages, either separately or continuously, in

which heavy reductions are made at each pass,

and ordinarily we use from 4 to 6 passes.

The cold rolling reduction, while it may uniformly be depended upon to give the type of orientation referred to as cold rolling orientation, must be carried on, however, in a particular way if the results attained by our invention are to follow. The material must not be allowed to rise in temperature at the various passes above a relatively fixed limit, since if this is done our optimum results will not be secured. We have noted that if the material, just after it leaves the pass in a rolling mill, has a temperature sufficient to cause water to boil on its surface, this material will not give optimum results. The water .est is not always definitive, since such factors as a film of oil frequently have an effect in preventing water from being heated up by the sheet as much as it would be if in direct contact therewith. It is, however, definitive to this extent, that with the chemical analyses given, or moderate departures therefrom, if the sheet is kept below the temperature of boiling water, the

results which we secure may be obtained. With steels of other formulae the operator will readily find the critical maximum temperature by examination of this product at the proper stage for the twinned condition and noting its presence or absence.

The particular reductions per pass which we have set forth in our exemplary embodiment, in view of the large radiating surface of the sheet, are not sufficient to carry the sheet to a high temperature; but if the heat is retained from pass to pass, the rise of temperature in the sheet may be cumulative. This we find it necessary to avoid. There are a number of ways of doing this. When rolling silicon steel strip by a plurality of passes through a single mill, it is advisable to give each coil a pass, and then set it aside to cool off, while the roller is rolling other coils. Ordinarily sufficient cooling so as to make the temperature ef-' fect substantially non-cumulative will be attained if the roller handles the material in batches, say, of 20 coils, and rolls these coils in rotation. Where it is desired to repeat rollings at once upon single coil, or where the material is to be rolled in a tandem mill, it is preferable to employ some artificial cooling means between 2 passes. Any artificial cooling means may be employed. It is convenient to lead the strip between passes through a bath of water, or other cooling medium, or to convey the strip between passes through an ordinary scrubber.

The keeping of the temperature of the strip low during the cold rolling reduction is vital if optimum results ale to be secured. It is one factor which enables us to depart from that type of orientation to which We have referred as cold rolling orientation, and also to secure a higher degree of preferred orientation. The reason for this is believed to be that: Whereas cold rolling orientation and the resultant thereof upon heat treatment, is not productive of anything more than the ordinary cold rolling orientation, it is possible to secure the different orientation to which we have referred, and to secure much more perfect selective orientation if a process of crystal twinning is caused to go on in material.

Thus in a two step process if the conditions are right, not only will the first cold rolling produce what is known as cold rolling orientation, but also there will be a degree of twinning of crystals initiated in the strip. Upon the subsequent intervening annealing, this twinned condition will increase in extent, and upon the final cold rolling and ultimate heat treatment, substantially all of the crystals will have assumed the desired preferred orientation by passing through a twinned stage. This condition, so far as we know, has never hitherto been attained, unless by accident, in accordance with any of the teachings of the prior art. Inasmuch as the twinning will not occur in material of this chemical analysis if the temperature is permitted to rise substantially above a temperature approximately that of boiling water, the necessity for the particular precautions which we have mentioned in the cold rolling step will immediately become apparent.

There is no way of which we are aware of securing substantially complete preferred orientation, except by allowing the material to pass through stages of crystal change of which the process of twinning is a part.

Having cold rolled the material in the first stage with the precautions mentioned, we next open anneal it at 1850 F., slit it if necessary,

and pickle it. This annealing is not critical as to temperature, but is primarily employed to bring the sheet'back to condition for further drastic cold work, and to cause the twinned nuclei to grow by accretion.

The next step is a second eold rolling stage to which the same precaution should be applied, and, in which in our exemplary embodiment, we cold roll the material, which is now 0.029 inch thick, in 6 to 10 passes to 0.013 or 0.014 inch in thickness. This amounts to a reduction of the order of 63%, but again may be departed from as set forth above. We have, in particular, secured excellent results with 70% of cold rolling, and even higher percentages at this stage; although again too high a reduction such as would destroy the desired crystal orientation should be avoided.

The final step is a heat treatment, without an intervening pickling. This is preferably a fiat anneal, and it may under some circumstances be carried on as an open anneal; but we prefer a box anneal, and for reasons which will be set forth hereinafter, we prefer a hydrogen anneal. In our exemplary embodiment we cut up the material at the end of the second cold rolling stage into sheets, coat these sheets with magnesium oxide as a separator, and anneal them fiat in a box for sixty hours at 2200 F., in an atmosphere of hydrogen with a dew point below -25 0., the

hydrogen being kept low in impurities. The final anneal not only has its appropriate eifect upon the crystal structure as indicated above, but also is of service in producing a material having an unusually low core loss. It will be competent to. precede the hydrogen anneal with a scaling anneal, if special steps are desired at this point for further carbon elimination. Our material is especially adapted for electrical transformer use since its permeability in the transformer range of inductions is exceptionally high, so that great economies can be made in transformer design; but it also increases the efliciency of the transformer, irrespective of its design, by reason of the low core loss which it has.

As'we have indicated above, we can carry on our process with more than two stages of cold rolling. We achieve good results, for example, starting with a hot rolled material 0.109 inch in thickness, giving it an initial heat treatment as described, pickling it, and then cold rolling it to 0.058 inch, giving it an open anneal at 1850 E, pickling it, cold rolling it to 0.023 inch, again giving it an open anneal at a temperature of 1850' R, pickling it, finally cold rolling it to 0.014 inch, and ultimately giving it the hydrogen anneal which we have described. The same precautions should be observed in this three step cold rolling process as have been mentioned in connection with the two step process described. Wefind that the three step process gives results comparable in all respects to the first exemplary embodiment set forth. Again, the particluar figures given are indicative rather than limitlns.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent, ls:-

1. A process of producing high permeability silicon steel which comprises box annealing silicon steel hot rolled starting pieces, cold rolling said pieces to produce a reduction therein of the order of 70%, while keeping the temperature of said pieces below approximately the boiling point of water, annealing said pieces and giving them a second cold rolling reduction, at similar low temperatures of the order of pproximately 60%, and finally heat treating sai pieces.

2. A process of producing high permeability silicon steel which comprises box annealing silicon steel hot rolled starting pieces, cold rolling said pieces to produce a reduction therein of the order of 70% while keeping the temperature of said piecesbelow approximately the boiling point of water, annealing said pieces and giving them a second cold rolling reduction, at similar low temperatures, of the order of approximately 60%, and finally heat treating said pieces, said final heat treatment being an anneal in hydrogen.

3. A process of producing high permeability silicon steel which comprises cold rolling hot rolled silicon steel starting pieces to produce therein a cold rolling reduction of substantially between 60 to 85%, while keeping the temperature of said pieces below approximately the boiling point of water, annealing said pieces and giving them a second cold rolling reduction, at similar low temperatures, of substantially between 40 to 65%, and finally heat treating said pieces.

4. A process of producing high permeability silicon steel which comprises cold rolling hot rolled silicon steel starting pieces to produce therein a cold rolling reduction of substantially between 60 to 85%, while keeping the temperature of said pieces below approximately the boiling point of water, annealing said pieces and giving them a second cold rolling reduction, at similar low temperatures, of substantially between 40 to 65%, and finally heat treating said pieces, said final heat treatment being an annealing treatment in hydrogen, low in impurities, and at approximately 2200 F.

5. A process of producing high permeability silicon steel which comprises box annealing silicon steel hot rolled starting pieces, cold rolling said pieces to produce therein a cold rolling reduction of substantially between 60 to 85% while keeping the temperature of said pieces below approximately the boiling point of water, annealing said pieces and giving them a second cold rolling reduction, at similar low temperatures, of susbtantially between 40 to 65 and finally heat treating said pieces, said final heat treatment being an annealing treatment in hydrogen, low in impurities, and at approximately 2200 F.

6. A process of making high permeability silicon steel which comprises starting with hot rolled silicon steel having a thickness of approximately 0.109 inch, cold rolling said material to approxilmately 0.058 inch while keeping the temperature